Type:

caption :string

From the PDF reference:
(Optional, button fields only) The widget annotation's normal caption which shall be displayed when it is not interacting with the user.
Unlike the remaining entries listed in this Table which apply only to widget annotations associated with pushbutton fields (see Pushbuttons in 12.7.4.2, "Button Fields"), the CA entry may be used with any type of button field, including check boxes (see Check Boxes in 12.7.4.2, "Button Fields") and radio buttons (Radio Buttons in 12.7.4.2, "Button Fields").

Type:

noToggleToOff :boolean

(Radio buttons only) If set, exactly one radio button shall be selected at all times; selecting the currently selected button has no effect. If clear, clicking the selected button deselects it, leaving no button selected.

Type:

radioIsUnison :boolean

(PDF 1.5) If set, a group of radio buttons within a radio button field that use the same value for the on state will turn on and off in unison; that is if one is checked, they are all checked. If clear, the buttons are mutually exclusive (the same behavior as HTML radio buttons).

Type:

boolean

readOnly :boolean

If set, the user may not change the value of the field. Any associated widget annotations will not interact with the user; that is, they will not respond to mouse clicks or change their appearance in response to mouse motions. This flag is useful for fields whose values are computed or imported from a database.

Type:

showWhenPrinted :boolean

(PDF 1.2) If set, print the annotation when the page is printed. If clear, never print the annotation, regardless of wether is is displayed on the screen.
NOTE 2 This can be useful for annotations representing interactive pushbuttons, which would serve no meaningful purpose on the printed page.